Preparation of Advanced CCMs for AMFCs by Amination and Cross-linking of the Precursor Form of the Ionomer

ABSTRACT

In an AMFC, in the formation of a CCM, the anode catalyst layer is selectively cross-linked while the cathode catalyst layer is not cross-linked. This has been found to provide structural stabilization of the CCM without loss of initial power value for a CCM without cross-linking.

RELATED APPLICATIONS

This application claims priority to U.S. provisional application Ser.No. 61/778,921, filed Mar. 13, 2013, which is incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

This invention relates to fuel cells and in particular alkaline membranefuel cells and apparatus and methods of stabilizing the CCM structureduring fuel cell operation.

BACKGROUND

The technology of alkaline membrane fuel cells (AMFCs) has beendeveloped to date with OH-ion conducting polymers (“ionomers”) with apoly{hydrocarbon} backbone. Such ionomers require significant wateruptake to achieve sufficient ionic conductivity. The structure andspecification of an AMFC is illustrated in, for example, US2010/0021777, entitled “Alkaline Membrane Fuel Cells and Apparatus andmethods for Supplying Water Thereto”, the entire contents of which areherein incorporated by reference

As the level of water uptake increases—likely over 50% by weight—theionomer typically loses mechanical integrity, resulting in morphologicalchanges and, in some cases, overall disintegration which causes strongloss of cell performance. The likelihood of such mode of failure isparticularly high on the anode side of the AMFC, where water isgenerated during cell operation, according to :

-   -   Anode process: 2H2+40H—=4H2O+4e    -   whereas the AMFC cathode runs on a water consuming process:    -   Cathode process: O2+2H2O+4e=4OH—    -   and is therefore much less likely to face a structural challenge        by buildup of excess liquid water.

To help stabilize the cell structure, cross-linking has been suggestedby one of the present inventors in a prior application as means forchemical bonding across the electrode/membrane interface. Thatapplication is U.S. Pub. No. US 2011/0300466 entitled “Chemical Bondingfor Catalyst/Membrane Surface Adherence in Membrane Electrolyte Fuelcells”, the entire contents of which are herein incorporated byreference.

The above application suggests that chemical bonding of a catalyst layerto a surface of an OH-ion conducting membrane or membranes isaccomplished by cross-linking of polymer components across an interfacebetween the catalyst layer and the membrane. According to the aboveapplication, both the anode side and the cathode sides of the catalystlayer-covered polymeric membrane (CM) are bonded by cross-linking withthe polymeric components of the catalyst layer, either at the interfacealone or with the cross-linking extending well into the catalyst layer.The methodology for cross-linking is disclosed in Paragraphs [0010]through [0016] of the above publication.

SUMMARY OF THE INVENTION

We describe in this application a comprehensive preparation of acatalyst layer—covered membrane (CCM) for an AMFC with the option tocross-link the ionomer in the catalyst layer on both sides, or only onone side of the CCM. The preparation starts from the CCM all inprecursor form, followed by amination including use of diamines forcross-linking and next followed by immersion in a base to generate theactive form of the CCM. We have found that selective cross-linking ofthe anode catalyst layer, while leaving the ionomer in the cathodecatalyst layer not cross-linked, provides the benefit of structuralstabilization of the CCM during cell operation while achieving initialpower output at least as high as that demonstrated for the CCM withoutcross-linking. In an alternative approach developed, the cross-linkedanode is prepared separately as a gas diffusion electrode (GDE) which iscompressed in the cell onto one side of an OH— conducting membrane,pre-covered by a cathode catalyst layer on its other side.

In one embodiment, an alkaline membrane fuel cell (AMFC) includes ananode electrode, a membrane electrolyte which I so configured to conducthydroxyl (OH—) anion, and the membrane has an interface which is inphysical contact with the anode electrode on a first surface of themembrane. The AMFC further includes a cathode electrode which is inphysical contact with the opposite surface of the membrane; both theelectrodes include a catalyst layer and a ionomer component of thecatalyst layer on the anode electrode is cross-linked to achievestructural stabilization with the loss of cell power.

In another aspect, the AMFC may be operated without a supply of waterfrom an external source.

In another aspect, the cross-linking of the anode side takes place bypreparation of a membrane catalyzed on one side with the polymer in bothmembrane and electrode in precursor form and the membrane and the singlecatalyst layer undergo a two-step conversion process to fuel cellactive, ionomer form (OH— form), using amination and cross-linkingagents in the first step of the process, and where the cathode catalystlayer is applied last on the second side of the membrane in fuel cellactive, ionomer form without crosslinking.

In yet another aspect, the cross-linking of the anode ionomer takesplace by preparation of an anode gas-diffusion electrode (GDE) with theanode catalyst layer applied to a gas-diffusion layer (GDL) in a processwhich involves cross-linking of the ionomer in the anode catalyst layerand the GDE so formed as compressed during cell building onto one sideof a membrane in ionic form, and where the membrane was pre-catalyzed onits other side by a cathode catalyst layer and the cell completed by asecond GDL adjacent the outer side of the cathode catalyst layer.

In a further aspect, a process is disclosed for chemical cross-linkingof an ionomer applied selectively to the anode side of a CCM in an AMFCwherein the cathode catalyst layer is retained in non-cross-linked form.

In another aspect, in the AMFC above described, the cross-linking agentmay be a diamine, or a mixture of diamines, which are applied during thestep of amination of the precursor, and where the diamines may serve, inmixture with momoamine or by themselves to achieve optimized combinationof structural stabilization and ionic and water transport.

In yet another aspect, the precursor may be selected based uponmaximized Ionic Equivalent Capacity (IEC). The polymeric component inthe catalyst layer may be a mixture of precursor form and active, ionicform.

I another aspect, the amination and cross-linking are applied to theprecursor form in a non-aqueous solvent, such as, by way of exampleonly, toluene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first set of steps in the present invention of amethod of preparation of an anode cross-linked CCM for an AMFC.

FIG. 2 illustrates a second set of steps in the present invention of amethod of preparation of an anode cross-linked GDE.

FIG. 3 illustrates in graphical form the effects of the presentinvention in maintaining AMFC resistance (HFR) near its initial valueover a period of time.

FIG. 4 illustrates in graphical form the effects of the presentinvention on power output over long periods of operation.

DETAILED DESCRIPTION

FIG. 1 describes schematically the preparation of a catalyst coveredmembrane (CCM) for an alkaline membrane fuel cell (AMFC), in which theionomer in the anode catalyst layer is cross-linked while the ionomer inthe cathode catalyst is left in non-cross-linked form. The stepsdescribed are: (i) preparation of a single-sided catalyzed membranewhere the anode catalyst layer and the membrane are joined while inprecursor form, (ii) the precursor in both the anode and membrane isconverted by amination to active ionomer form, introducing also, in thesame step, cross-linking of the ionomer in the anode and in themembrane, typically by use of diamines*, (iii) hydroxylation of theionomer and (iv) attaching the cathode catalyst layer in ionomer form,to the other side of the membrane to complete the half cross-linked CCM,designated by us as “½XCCM”. The chemical processes involved inconversion to active form by amination and cross-linking by use ofdiamines, are described by chemical equations given in theabove-mentioned U.S. Pub. No.: US 2011/0300466.

FIG. 2 describes schematically an alternative preparation of a catalystcovered membrane (CCM) for an alkaline membrane fuel cell (AMFC), inwhich the ionomer in the anode catalyst layer is cross-linked while theionomer in the cathode catalyst is left in non-cross-linked form. Thesteps described are (i) preparation of a gas-diffusion electrode wherethe anode catalyst layer is deposited on gas-diffusion layer while inprecursor form, (ii) the precursor in both the anode is converted byamination to active ionomer form introducing also, in the same step,cross-linking of the ionomer in the anode, typically by use ofdiamines*, (iii) hydroxylation of the ionomer and (iv) attaching the GDEto the OH— ion conducting membrane with the cathode catalyst layer inionomer form pre-attached on the other side to complete the halfcross-linked CCM, designated by us as “½XCCM”. The chemical processesinvolved in conversion to active form by amination and cross-linking byuse of diamines, are described by chemical equations given in theabove-mentioned U.S. Pub. No.: US 2011/0300466.

FIG. 3 illustrates the beneficial effect of selective cross-linking ofthe ionomer on the anode side in maintaining the AMFC resistance (HFR)near its initial value after hundreds of hours of operation. The topcurve shows the significant rise of the AMFC resistance over time whenno cross-linking is used and the bottom curve shows the much smallerrise of HFR with operation time following cross-linking on the anodeside alone.

FIG. 4 illustrates the beneficial effect of selective cross-linking onthe anode side of the CCM in an AMFC on the power output over operationtimes of the order of hundreds of hours. Anode side cross-linking isseen to increase the initial power and lower the rate of power loss withtime, as compared with the case of no cross-linking.

Above, we have demonstrated that the modes of preparation ofmembrane/electrode assemblies for AMFCs involving cross-linking on theanode side, according to either of the techniques shown in FIG. 1 andFIG. 2, have achieved significant stabilization of the CCM vs. CCMsprepared with no cross-linking of the ionomer in the catalyst layers.The evidence for a more robust structure was the stabilization of theohmic (HFR) resistance of CCMs with cross-linked ionomer on the anodeside over hundreds of hours of cell operation. This was in significantcontrast to continuous rise of the HFR over similar length of operationtimes, observed consistently when no cross-linking was introduced, ascan be seen in FIG. 3.

Further, the inventors herein observed that stabilization of thecatalyst layer structures and any formation of interfacial bonds bycross-linking on the cathode side of the CCM, did not contributesignificantly further to stabilization of the cell HFR and,consequently, did not provide significant further stabilization of thecell power over operation time. It was concluded that ionomercross-linking may be of critical importance on the anode side of thecell because this is the site of water formation in the AMFC and,consequently, the morphology of the anode catalyst layer in the AMFC isvulnerable to over-swelling. In contrast, on the AMFC cathode suchover-swelling is highly unlikely, as the AMFC cathode process involveswater consumption, rather than water generation.

Targeting both ionomer phase stabilization and catalyst layer/membraneinterfacial bonding, cross-linking needs to be achieved across the anodecatalyst layer and into the membrane. For that purpose, a CCM was madestarting with an anode catalyst layer applied on one side of a membrane,where the polymer in both the membrane and the anode catalyst layer isin precursor (non-ionic) form. A mixture of monoamines and diamines, ordiamines alone, then converts the polymer to ionic form, whilecross-linking it at the same time. Following application next of anon-cross-linked cathode catalyst layer on the other side of themembrane, a “half cross-linked CCM” was formed, with the ionomer on theanode side only being cross-linked, while leaving the ionomer in thecathode catalyst layer not cross-linked and, thereby, facilitatingunhindered water transport to and within the cathode. This preparationof a “half-cross-linked” CCM was described above and illustratedschematically in FIG. 1. The test results for such “half-cross-linked”CCMs, confirmed that it was indeed specifically the anode catalyst layerthat required stabilization by cross-linking, as stabilization of cellresistance (HFR) over long operation times was well achieved by suchselective cross-linking confined to the anode side only, as shown inFIG. 3. Thanks to stabilization of the HFR in cells with the“half-cross-linked” CCM, the power density of such cells fell to lesserdegree with cell operation time, as illustrated in FIG. 4.

In looking for the best type of ionomer precursor for use on the anodeside of an AMFC, in which such precursor is to be functionalized andcross-linked to achieve the final form of the CCM, the inventors havenoted that some restrictions of water transport in the AMFC anode aretypically observed following cross-linking. It was discovered that theway to achieve anode stabilization by cross-linking at negligiblepenalty of water and ion transport in the cross-linked anode, is to usea precursor of the highest possible ionic site concentration, i.e., ofhighest IEC (Ionic Equivalent Capacity). Specifically, ionomers of IECvalues which would normally exhibit instability following long termimmersion in warm water (e.g., IEC close to 4 meq/gr), are renderedsufficient stability by optimized level of cross-linking, whilemaintaining, following such cross-linking, sufficient water and ionmobility thanks to the high concentration of ionic sites. The selectionof a very high-IEC precursor to achieve good structural stabilizationwithout loss of water and ionic transport rates, is, therefore, animportant part of our discovery.

In addition to the choice of most suitable ionomer precursor for thecombined process of functionalization and cross-linking, other detailsof the formulation and the conditions of combinedamination/cross-linking have been developed as part of this discovery.One option discovered is to use only di-amine (no mono-amine) to achieveboth functionalization and cross linking. Introduction of this lastapproach has special value in enabling to work with amines of lowvolatility (diamines) without the need to use highly volatile monoamineswhich are notorious for their bad odor. A single diamine or a mixture oftwo diamines were found to work best in various cases. The choice ofsolvent is another important factor and use of various non-aqueoussolvents in the functionalization and cross-linking process has somespecial merit in swelling the polymer during the process. It has beenfound, for instance, that while replacing trimethylamine (TMA) with thediamine DABCO for amination, the best solvent to achieve crosslinking istoluene, and not water as regularly used. Using toluene as solventallows better solvation of the polymer achieving then optimizedcrosslinking in the polymer backbone. Toluene also was found to be thebest solvent for amination and crosslinking while using a mixture ofordinary diamine (like hexanetetramethyldiamine) and DABCO for bothamination and crosslinking steps.

In yet another formula developed to maximize transport rates followingcross-linking, a high IEC ionomer was mixed with a high IECionomer-precursor before functionalization of the precursor inducedtogether with cross-linking, by use of some mixture and concentrationsof mono- and di-amines. In this way, a fraction of the ionicmaterial—the high IEC ionomer—assists in achieving higher watertransport rates, as, unlike the precursor, it will not be cross-linkedwhile being functionalized. At the same time, the non-cross-linked,water accommodating, high IEC ionomer, is given protection fromdissolution by entrapment inside a network of the cross-linked ionomergenerated from the ionomer precursor.

All the above described features of the ½ XCCM preparation routinedeveloped, that were aimed to sustain highest water and ion transportrate following cross-linking were successful in maintaining theimportant capability to operate the AMFC stacks without any supply ofwater from an external source. This is considered a key achievement, asthe trade-off between stabilization by cross-linking and loss oftransport rate typically associated with it is not easy to resolve inprinciple.

An alternative approach to achieve an AMFC with a cross-linked anode isto prepare the cell with a gas diffusion anode electrode (GDE) which iscross-linked and attached to a membrane with the other side of themembrane pre-coated by a cathode catalyst layer (see FIG. 2). Such a GDEanode is prepared by application of the anode catalyst layer onto a gasdiffusion layer (GDL) using spraying or printing, followed by treatmentwith a cross-linking agent, for example di-amines, to achieve anodecross-linking. The membrane with a cathode catalyst pre-applied to oneside, is prepared without cross-linking, and the cross-linked anode GDEis attached to the free side of the membrane by mechanical compressionto form the full cell (see FIG. 2). A possible advantage of the separatepreparation of a cross-linked, anode GDE, is in facilitating watertransport from such anode into the membrane, as the cross-linkingprocess is now confined to the bulk of the anode catalyst layer with the“seam” between the anode catalyst layer and the membrane left notcross-linked. Thereby, water crossing is facilitated from the anode intothe membrane, towards the water consuming cathode.

What is disclosed here is believed to be an original approach tostructural stabilization of AMFC catalyst layers and CCMs, based onamination and cross-linking of a CCM in precursor form and, within thistechnical approach, CCMs for AMFCs where cross-linking is confined tothe anode side, thereby allowing significant stabilization of thevulnerable anode catalyst layer while keeping the water transportunhindered in the cathode. The latter approach allows to achieve,simultaneously, significant stabilization of the cell resistance vs.non-cross-linked CCMs and the maintenance (if not increase) of theinitial power level seen for non-cross-linked (and less stable) CCMs.

1. An alkaline membrane fuel cell comprising: an anode electrode; amembrane electrolyte configured to conduct hydroxyl (OH—) anion, themembrane having an interface in physical contact with the anodeelectrode on the first surface of the membrane; a cathode electrode inphysical contact with second opposite surface of the membrane; the anodeelectrode and the cathode electrode each having a catalyst layer;wherein an ionomer component of the catalyst layer on the anodeelectrode is cross-linked to achieve structural stabilization withoutloss of cell power
 2. The fuel cell in claim 1 operated with no supplyof water from an external source
 3. The fuel cell in claim 2 whereincross-linking of the anode side of the cell takes place by preparationof a membrane catalyzed on one side with the polymer in both membraneand electrode in precursor form and the membrane and single catalystlayer undergo a two-step conversion process to fuel cell active, ionomerform (OH— form), using amination and cross-linking agents in the firststep of this process, and where the cathode catalyst layer is appliedlast on the second side of the membrane in fuel cell active ionomer formwithout crosslinking.
 4. The fuel cell in claim 2 wherein cross-linkingof the anode ionomer takes place by preparation of an anodegas-diffusion electrode (GDE) with the anode catalyst layer applied to agas-diffusion layer (GDL) in a process which involves cross-linking ofthe ionomer in the anode catalyst layer and the GDE so formed iscompressed during cell building onto one side of a membrane in ionicform, wherein the membrane was pre-catalyzed on its other side by acathode catalyst layer and the cell completed by a second GDL adjacentthe outer side of the cathode catalyst layer.
 5. A process for chemicalcross-linking of an ionomer applied selectively to the anode side of aCCM in an AMFC wherein the cathode catalyst layer is retained innon-cross-linked form
 6. The fuel cell of either claim 3 wherein thecross-linking agent is a diamine, or mixture of diamines, applied duringthe step of amination of the precursor, where the diamines can serve inmixture with monoamine, or by themselves, to achieve optimizedcombination of structural stabilization and ionic and water transport 7.The fuel cell of claim 6 wherein the precursor is chosen based onmaximized IEC.
 8. The fuel cell of claim 1 wherein the polymericcomponent in the catalyst layer is a mixture of precursor form andactive, ionic form.
 9. The fuel cell of claim 2 wherein amination andcross-linking are applied to the precursor form in non-aqueous solvent.10. The fuel cell of claim 9 wherein the non-aqueous solvent is toluene.11. The fuel cell of claim 4 wherein the crosslinking agent is adiamine, or mixtures of diamines, applied during the step of aminationof the precursor, where the diamines can serve in mixture withmonoamine, or by themselves, to achieve optimized combination ofstructural stabilization and ionic and water transport.